before its large eruption and afterwards, in the valley of Antofagasta de la Sierra and may have continued to less than ten thousand years ago. The positions of the exact vents are controlled by recent fault systems in the region. Since about 10 million years ago, the area has been subject to reverse faulting which has disrupted the basement along north–south lines, forming a rift valley that also stretches from north to south. The magma erupted by the Galán system was likewise channelled along such fault systems, and neighbouring volcanoes were similarly influenced by them; the fault systems at Galán proper are known as the Diablillos-Galán faults. Another major lineament in the area is the Archibarca lineament, which is formed by a strike-slip fault that extends from the northwest to the southeast in the region and which intersects the Diablillos-Galán faults at the location of the caldera. Composition Galán has erupted mainly potassium-rich dacitic to rhyolitic rocks that are often called rhyodacitic, and which reflect a calc-alkaline suite. Each ignimbrite has usually a uniform composition but there is some variation between individual ignimbrites; for example older rocks contain amphibole and younger rocks instead sanidine. Minerals contained in the eruption products include allanite, apatite, biotite, hornblende, ilmenite, magnetite, orthopyroxene, plagioclase, quartz, sanidine and zircon. Hydrothermal alteration has left calcite in some rocks. Trace element patterns are distinct in the Galán ignimbrite in comparison to the Toconquis Group rocks. The formation of the Galán magma has been explained with melting of lower crustal rocks under the influence of rising basaltic magmas that supplied the heat needed for the melting processes, and which also directly contributed to magma formation through mixing events. Further metasomatism in the crust and fractional crystallization processes completed the magma genesis process. Probably under the influence of larger scale tectonics, magma that accumulated into a mid-crustal mush zone is eventually transferred into shallow magma chambers at depths of ; recharge events where deep magma entered the shallow magma bodies may have triggered eruptions at Galán. After eruption, a leftover pluton would have been generated inside the crust. Based on the presence of two separate populations of pumice in the Galán ignimbrite it has been inferred that there were two types of magma in the magmatic system during the Galán eruption, a larger volume of so-called "white" magma and a "grey" magma which was injected into the "white" magma pool